Integrated circuits, such as microprocessors and memory devices, include many metal-oxide-semiconductor field-effect transistors (MOSFETs), which provide the basic switching functions to implement logic gates, data storage, power switching, and the like.
Power MOSFETs have typically been developed for applications requiring power switching and power amplification. In a power MOSFET, it is desirable to reduce the resistance of the device during conduction (Rds-on) and improve its breakdown voltage (BV). In a transistor, much of the breakdown voltage (BV) is supported by a drift region, which is lowly doped in order to provide a higher breakdown voltage BV. However, the lowly doped drift region also produces high on-resistance (Rds-on). In other words, the on-resistance (Rds-on) and the breakdown voltage (BV) are in a tradeoff relation to each other. In fact, the on-resistance Rds-on is proportional to BV2.5. That is, the on-resistance (Rds-on) increases dramatically with an increase in breakdown voltage (BV) for a conventional transistor.
Superjunction device configurations have been employed to provide a way to achieve low on-resistance (Rds-on), while maintaining a high off-state breakdown voltage (BV). Superjunction devices include alternating p-type and n-type doped columns arranged in parallel and connecting to each other in the drift region. The alternating p-type and n-type columns are in substantial charge balance. When a reverse-bias voltage is applied to between the drain and the source, these columns deplete one another (i.e., laterally) at a relatively low voltage so as to withstand a high breakdown voltage in the vertical direction. The on-resistance (Rds-on) for a superjunction device increases in direct proportion to the breakdown voltage BV, which is a much less dramatic increase than in a conventional semiconductor structure. A superjunction device may therefore have significantly lower on-resistance (Rds-on) than a conventional MOSFET device for the same high breakdown voltage (BV) (or conversely may have a significantly higher breakdown voltage BV than a conventional MOSFET for a given on-resistance Rds-on).
Shielded gate trench (SGT) MOSFETs are another type of power MOS devices. They are preferred for certain applications over conventional MOSFETs due to their advantageous characteristics, including reduced gate-to-drain capacitance Cgd, reduced on-resistance Rds-on, and increased breakdown voltage BV of the transistor.
It is within this context that embodiments of the present invention arise.